Magnesium ions are polyvalent ions, and accordingly, the electric capacity of magnesium per unit volume is high. In addition, magnesium has a higher melting point and is safer compared to lithium, magnesium resources are relatively evenly distributed on earth, and magnesium is inexpensive because the resources are abundant on earth. Therefore, a magnesium ion battery adopting metallic magnesium as a negative electrode is drawing attention as a next-generation battery replacing a lithium ion battery.
However, in the magnesium ion battery adopting metallic magnesium as a negative electrode, magnesium reacts with an electrolytic solution owing to its high reducibility, and as a result, a passive film is formed on the electrode surface. Consequently, reversible dissolution-precipitation of magnesium is hindered, which makes it difficult for a negative electrode reaction to occur.
As an electrolytic solution which does not form the passive film, an electrolytic solution containing a Grignard reagent RMgX (R represents an alkyl group or an aryl group and X represents chlorine or bromine) as an electrolyte is known, and this electrolytic solution has been confirmed to enable reversible dissolution-precipitation of magnesium. However, because the Grignard reagent RMgX is strongly basic, the electrolytic solution has a safety issue. Furthermore, unfortunately, the electrolytic solution is impractical because oxidative stability thereof is low.
Therefore, an electrolytic solution having improved safety and performance has been developed by mixing the strongly basic Grignard reagent or an organic magnesium reagent with a Lewis acid having boron or aluminum.
For example, Aurbach and others have reported that a tetrahydrofuran solution of an electrolyte Mg(AlCl2BuEt)2 in which aluminum is bonded to a halogen enables magnesium to be dissolved-precipitated with high efficiency and has high voltage resistance (Non-Patent Literature 1, 2, and the like). However, this solution has an oxidative decomposition potential of about +2.5 V for magnesium, and hence the oxidative stability thereof is still not high enough for the solution to be a substitute for a lithium ion battery.
There is also an electrolytic solution known to use boron as a Lewis acid, enable dissolution-precipitation of magnesium as well, and exhibit high voltage resistance. For example, Muldoon and others have reported an electrolytic solution adopting [Mg2(μ-Cl)3.6THF][BR4] as an electrolyte (Patent Literature 1 and the like). Herein, R each represents an alkyl group or an aryl group which may be substituted with an alkyl group, an alkoxy group, a cyano group, or a nitro group. According to the literature, even though any of a platinum electrode and a stainless steel electrode is used as a working electrode for magnesium, approximately the same level of oxidative stability is realized, but the oxidative stability does not reach 3 V for magnesium.
As another example of the boron-based electrolyte, Mohtadi and others have reported Mg(BX4)2 (X represents hydrogen, fluorine, or alkoxy group) in US 2014038061 (Patent Literature 2). However, the literature just describes that in a case where glassy carbon is used as a working electrode, the boron-based electrolyte remains stable up to 2.3 V for magnesium. In addition, Mohtadi and others have also reported an electrolytic solution containing a carboranyl magnesium electrolyte and realized oxidative stability higher than 3 V in the electrolytic solution containing boron (Patent Literature 3). However, the electrolytic solution has an issue of practicality because the electrolyte has a special structure and uses raw material lacking generality.